As described in prior patents, including U.S. Pat. Nos. 6,183,375, 6,572,488 and 6,997,056 issued to Weiss et al., the golfing industry has recognized that substantially all golf club shafts exhibit some degree of asymmetry in their physical characteristics. The industry has also generally recognized that substantially every golf club shaft exhibits at least one orientation in which, when the shaft is clamped at its grip end and displaced at the tip end, the resultant vibration of the shaft remains substantially planar. That is, the oscillation of the shaft remains substantially in a single plane, with the tip of the shaft vibrating back and forth substantially along a line. Although a shaft may exhibit nearly planar oscillation at more than one axial rotation position, there is typically a single axial rotation position at which the oscillation is most planar and stable. This single plane is referred to as the principal planar oscillation plane (PPOP). The patents listed above and others have thoroughly documented the benefits associated with consistently aligning the principal planar oscillation plane of golf clubs within a set of clubs.
Asymmetry in golf club shafts can result from nonsymmetrical cross sections (shafts whose cross sections are not round or whose wall thicknesses are not uniform), shafts that are not straight, or shafts whose material properties vary around the circumference of the shaft cross section. Recognizing that it is substantially impossible to build a perfectly symmetric golf club shaft, the desired objective is to minimize inconsistencies from club to club and from set to set. Thus, if possible, it is desirable to analyze each golf club shaft in a set of golf clubs to understand its asymmetric bending or twisting behavior and construct the golf clubs in the set to maximize consistency from club to club and/or from set to set.
In addition to determining the principal planar oscillation plane of a shaft, it is also desirable to determine the torsional stiffness of the shaft, asymmetries in the stiffness of the shaft about its circumference, the length of the shaft, the amount of “droop” in the tip of the shaft when the grip end is secured, the straightness of the shaft (run-out), shaft relative stiffness, and modal vibrational frequencies of the shaft. Heretofore, there has been no fast and easy way to determine with consistency these parameters of a golf club shaft that would allow manufacturers or others to predict the performance of a golf club shaft.
What is needed, therefore, is a method and apparatus for quickly, automatically, and reliably determining all of the above-listed characteristics of a golf club shaft and providing data regarding these characteristics to shaft and club manufacturers and others.